No mistakes; just instructions. Basically, RVM tells you that Ruby 1.9.3 has dependencies that need to be installed before continuing. You can install these dependencies using a package manager such as aptitude (Linux Ubuntu) or Homebrew (Mac).
In your case you will need Homebrew since you are on a mac.
If you do not have a homegrown yet, go here to install it. Once it is installed, install the missing dependencies. In your case:
$ brew install libksba
Now let's get started with installing Ruby 1.9.3:
$ rvm install 1.9.3
Please feel free to post if you need further assistance.
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